1. Field of the Invention
The present invention relates to an image heating apparatus that is the optimum one used as a fixing device of an image forming apparatus such as a copying machine, a printer, or a facsimile device and, more particularly, to an image heating apparatus utilizing the principle of electromagnetic induction.
2. Related Background Art
An image heating apparatus in an image forming apparatus is an apparatus for fixing a toner image on the surface of a recording material as a permanent fixed image by heating, the toner image being formed on the surface of the recording material in a direct or indirect (transfer) manner by using a toner (visualizing agent) composed of a heat melting resin through proper image formation processing means of electrophotographic, electrostatic recording, or magnetic recording in an image forming portion of the image forming apparatus.
Hitherto, as such an image heat fixing apparatus, a heat roller type one and a film heating type one have been put into practical use. In recent years, an apparatus of an electromagnetic induction heating type has been proposed. Particularly, an image heating apparatus of the electromagnetic induction type has high heat transfer efficiency from a heat source to the toner because a rotating element itself which is come into contact with the recording material can be allowed to generate heat. The apparatus is received attention.
The electromagnetic induction heating type apparatus uses an electromagnetic induction heat generating element as a heating element, operates magnetic field generating means so that a magnetic field acts on the electromagnetic induction heat generating element to generate an eddy current in the electromagnetic induction heat generating element, and thermally fixes the toner on the surface of the recording material as a heating material by Joule heat based on the eddy current.
In Japanese Patent Publication No. 5-9027, disclosed is an apparatus for electromagnetic-induction heating a fixing roller made of a ferromagnetic material. A heating position can be close to a fixing nip portion. Such an apparatus realizes a fixing process with high efficiency that is higher than that of the heat roller type apparatus utilizing a halogen lamp as a heat source.
In Japanese Patent Application Laid-Open No. 4-166966, disclosed is an electromagnetic induction heating type fixing apparatus using a film-shaped rotating element in which the heat capacity is reduced. Since the film itself, which is come into contact with toner, generates heat and the heat capacity is smaller than that of the aforementioned fixing roller, the apparatus has remarkably high efficiency.
In each of the above heating type apparatuses, when a plurality of recording materials are continuously printed, a temperature in a non-sheet passing portion rises. Particularly, in the apparatus using the film-shaped rotating element whose heat capacity is reduced, heat conduction in the longitudinal direction (longitudinal direction of the fixing nip portion) is small. When small-sized recording materials are continuously passed, excess temperature rise occurs in the non-sheet passing portion. Accordingly, there is such a problem that the operating life of a film or a pressurizing roller is deteriorated.
As a method of solving the above problem, there is considered a method whereby in case of using a small-sized recording material, a cooling time for a fixing film is set by extending a sheet feeding interval to reduce the throughput (the number of passing sheets per unit time). In order to obtain the necessary cooling time, there is such a problem that the inherent image forming speed of the apparatus is remarkably deteriorated.
As for the electromagnetic heating system, there is considered a method of preventing the temperature rise in the non-sheet passing portion in such a manner that the magnetic field generating means is divided and arranged to the fixing nip portion in the longitudinal direction, and the divided means are individually operated to perform partial excitation so that a heating area is limited. The apparatus having a large number of magnetic field generating means has a complicated structure and has many problems such that it is necessary to adjust excitation phase of each of the plurality of magnetic field generating means. There is also such a problem that the cost is increased.
The present applicant has proposed such an art that one excitation coil allows a fixing rotating element to generate heat in the longitudinal direction and a conductor for demagnetization is arranged in the non-sheet passing portion to suppress heat generation in the non-sheet passing portion of the rotating element in low costs in Japanese Patent Application Laid-Open No. 9-171889.
According to the present invention, the above art is further improved to provide an apparatus which is applicable to recording materials of various sizes and various thicknesses.